1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus which projects and transfers a circuit pattern drawn on a reticle (mask) onto, for example, a wafer via a projection optical system has conventionally been employed to fabricate a semiconductor device using photolithography (printing). Since this exposure apparatus is required to precisely transfer the pattern on a reticle onto a wafer with a predetermined magnification (reduction ratio), it is important to use a projection optical system with a good imaging performance and small aberration.
In recent years, along with further progress in the micropatterning of semiconductor devices, a pattern with a minimum line width that exceeds the general imaging performance of an optical system has come to be often transferred, and therefore the transferred pattern is becoming sensitive to the aberration of the optical system. This makes it necessary to suppress the allowable amount of residual aberration of the projection optical system to 10 mλ or less in rms value and, recently, to several mλ in rms value.
Under the circumstances, even when the aberration of the projection optical system is guaranteed, an amount of aberration as small as about several mλ may change with time. To cope with this problem, it is demanded to measure the optical performances (particularly the wavefront aberration) of the projection optical system while it is mounted (built) in the exposure apparatus, that is, while it is ready for use in actual exposure.
To meet this demand, there have been proposed exposure apparatuses which mount interferometers such as a point diffraction interferometer (PDI), line diffraction interferometer (LDI), and lateral shearing interferometer each of which is used to measure the wavefront aberration of a projection optical system. These exposure apparatuses are disclosed in Japanese Patent Laid-Open Nos. 2005-244126, 2006-073697, and 2006-108597.
The imaging performance of the exposure apparatus is known to also depend on an effective light source distribution formed by an illumination optical system. For example, if a plurality of exposure apparatuses use different effective light source distributions, they form patterns with different line widths even when the same reticle is exposed. To cope with this problem, it is also demanded to measure an effective light source distribution formed by the illumination optical system on the exposure apparatus. The effective light source distribution can be measured by receiving a light beam, which emerges from a pinhole PH that has a diameter of several micrometers to several tens of micrometers and is inserted between an illumination optical system and a projection optical system PS, via the projection optical system PS by a sensor LS arranged on a wafer stage as shown in FIG. 10. FIG. 10 is a view for explaining the conventional mechanism of effective light source distribution measurement.
It is also demanded to measure the light distribution on the pupil plane of the projection optical system upon illuminating a reticle under an illumination condition under which a reticle for use in actual exposure is exposed, that is, the distribution (diffracted light distribution) of light diffracted by the reticle pattern. Measuring the diffracted light distribution on the entire reticle surface makes it possible to predict a change in the aberration of the projection optical system due to exposure heat with high accuracy.
Unfortunately, when measurement patterns (of a substrate) and sensors dedicated to respectively measuring the wavefront aberration, effective light source distribution, and diffracted light distribution are mounted (arranged) on a wafer stage, the weight on the wafer stage increases, resulting in a decrease in stage performance and an increase in stage size. In addition, when a large number of sensors are arranged on the wafer stage, heat generated by these sensors is accumulated and acts on the wafer stage, resulting in a further decrease in stage performance.